This invention relates generally to analog-to-digital converters and more particularly to converters of such type which provide a "reflected binary" or Gray code digital signal representation of the level of an analog signal applied thereto.
As is known in the art, one type of such a "Gray code" analog-to-digital converter includes a series of cascaded encoding stages, each one thereof providing a digital output and a residue analog output signal, the latter such signal being passed as the input to the next stage. Each one of the stages has a V-shaped transfer characteristic between the analog input signal applied thereto and the residue analog output signal produced at the output thereof. Such V-shaped transfer characteristic is disposed symmetrically about the midpoint of the dynamic operating range of the stage. The V-shaped transfer characteristic is generally formed by using a network having a nonlinear switching element, such as a diode, and biasing means for enabling such switching element to switch at the midpoint of the dynamic operating range mentioned above.
While such analog-to-digital converters have been found to be adequate in some applications, such converters have not been found adequate in other applications as where the analog signal applied to the converter has a large dynamic range of level variation, with a mean level disposed at the midpoint of the operating range of the converter. For example, in a radar system wherein digital processing is used, an analog-to-digital converter is generally used to digitize a bipolar video frequency signal. Such signal may have a large dynamic range of level variation, with a mean level of zero volts. According to the prior art described above the V-shaped transfer characteristic of each one of the encoding stages is disposed symmetrically about the zero volt level. It is first noted that in any practical application, the accuracy in generating a discontinuous transfer characteristic, such as a V-shaped transfer characteristic, is particularly difficult at the point of such discontinuity. It is next noted that switching errors are generally more likely to occur in response to relatively low signal levels. Therefore, with an analog-to-digital converter as described above, the point of discontinuity is disposed in the small signal level region of the operating range of such converter. The residue analog voltage produced at the output of any one of the initial stages may be in error. Any such error then is propagated erroneously through successive stages, thereby adversely affecting the accuracy of the converter.